Not applicable.
1. The Field of the Invention
The present invention relates to the field of hand-held power tools, and in particular safety aspects thereof.
2. The Relevant Technology
Hand operated power tools, of the type shown in European Patent No 0993919, EP Application No 95934753.5, and German Patent No 3239986 all have certain similarities. They each have a base plate which presses flat onto the work surface prior to and during the cutting operation. The cutting element is eased into the work piece by pressing the handle in a direction perpendicular to and towards the work surface, whilst the handle remains at a constant angle. The result is that the base plate and/or guard slides with respect to the handle.
The existing method used to achieve this sliding action is to attach two round rods onto the base plate, which are perpendicular to it. These slide in round bearings attached to the main case.
This system works well, but has the disadvantage of requiring a substantial base plate onto which the rods are mounted, in order to attain sufficient strength and rigidity. The rods also require very accurate positioning in both their location with respect to each other and their angle. If this positioning is poor the base plate will stick or wobble.
Both European Patent No 0 993 919 and European Patent Application No 95934753.5 show that increased safety is achieved if the base plate has four sides perpendicular to it, which prevent the user from coming into direct contact with the blade. These sides represent a guard which slides over or in the main case during operation. If this method is employed, an easier and cheaper method for achieving the sliding action is to attach bearing material to either the main case or the guard, and allow the other member to slide on this.
Japanese Utility Publication No 53-2786 refers to a hand-held tool whose blade may be surrounded by a protective guard housing. This housing is able to slide along one axis by means of guiding pins held within channels within this housing. However, this prior system has the disadvantage of these pins being prone to becoming jammed in the channels due to the direct contact with the inner surface of the channel. This device has only point contact within the sliding means increasing the likelihood of jamming. Poor angular location of the guard may also result from this direct, inflexible contact.
It is one object of the present invention to provide hand operated power tools with simple but effectively located blade guard.
According to a first aspect of the present invention, there is provided a guard for use with hand operated power tools, said guard being dimensioned to fit over and around the operating head of said power tool and being a sliding fit thereon, and wherein slider bearings are mounted between the guard and said operating head of the tool, as to maintain alignment between the guard and head over the operating range of movement of the guard.
Preferably, four such slider bearings are provided spaced around the periphery of the guard.
Preferably the slider bearings comprise co-operating tongue and groove formations on the guard and operating head or vice versa.
Preferably also, the slider bearings are made of or coated with a material with a relatively low co-efficient of friction, such as PTFE, polyacetal or polyamide.
Preferably also the guard is resiliently biased towards a first position in which a cutting blade of the tool is covered.
Typically, the guard features a position indicator scale and depth adjuster to enable the depth of the guard to be set to a known value.
Optionally, indication markings are also provided to show the chord length of the blade.
European patent applications numbers EP 99307894.4 and EP 95934753.5 show a method of indicating the chord length of exposed cutting blade by using an arc marked on the main body, which intersects with vertical lines marked on the movable guard. The chord length will vary depending on the setting of the depth adjuster, which limits the movement of the guard and thus the amount of exposed cutting blade. Having indication is useful when cutting out as is shows where the cut starts and finishes.
With this form of indication the chord length of exposed cutting blade, for any given depth, cannot be seen until the guard has been retracted as far as the depth adjuster will allow. That is, it is only possible to determine the foremost and/or rearmost portions of the cutting blade when the blade is fully extended. Therefore, in the prior art, it is difficult for an operator to use the indication for positioning the tool, prior to cutting. This is a problem when cutting out because the tool should only be fed in a forwards direction, whilst cutting, and requires positioning exactly at the start of the cut.
It would be desirable to have a means for indicating the position at which the blade will begin cutting before the tool is depressed and the blade is exposed.
According to a second aspect of the present invention, there is provided a guard for use with hand operated power tools, said guard being dimensioned to fit over and around the operating head of said power tool and being a sliding fit thereon, said guard being movable relative to said tool between a first position in which a cutting blade of the tool is covered, and a second position in which the cutting blade is exposed, wherein the guard is provided with indication marks for indicating to the user the foremost point at which the cutting blade protrudes from the guard when in both first and second positions.
The guard may be provided with a depth adjuster for controlling the maximum extension of the cutting blade from the guard base. The depth adjuster is preferably provided with an indicator scale to enable the depth of the guard to be set to a known value.
The indication markings on the guard may correspond directly to the indicator scale on the depth adjuster.
The indication markings preferably indicate the rearmost point at which the cutting blade protrudes.
A further drawback of hand operated electric circular saws and disk cutters of the prior art is that they can be prone to blade jamming which can occur if the cutting blade strikes a harder object than it is intended to cut, if the bade is twisted in the cut or if the tool is fed in the reverse direction. Blade jamming can cause damage to both the tool and the operator by loss of control.
According to a third aspect of the invention there is provided a hand operated power tool comprising a rotary cutting blade, a driving shaft, and a retaining washer, said cutting blade being urged towards the driving shaft by said retaining washer so that the blade and shaft rotate together, wherein the retaining washer is frangible and substantial restriction of the rotation of the cutting blade causes the retaining washer to break, thereby releasing the blade from being urged towards the driving shaft.
The retaining washer may be attached to the driving shaft by a screw extending through a hole in the retaining washer and the cutting blade.
Preferably, a substantial restriction of the rotation of the cutting blade causes a tightening of the screw in the driving shaft, said tightening causing the retaining washer to break.
The retaining washer may comprise an outer region of a first thickness, and an inner region of a second thickness, the second thickness being less than the first thickness. Preferably a fixing screw abuts the inner region, and the outer region abuts the cutting blade.
The retaining washer may be made from a brittle material, for example die cast zinc.